In recent years, plasma treatment techniques have widely been used to treat objects in an etching process and a deposition process in the production of semiconductors and liquid crystals. A reactive and corrosive gas containing a halogen element, such as fluorine or chlorine, has frequently been used in an etching process and a deposition process. High corrosion resistance is therefore required for members having a corrosion-resistant surface, which comes into contact with a corrosive gas or its plasma in treatment apparatuses, such as etching apparatuses and coating apparatuses, for use in the production of semiconductors and liquid crystals. Members having such a high corrosion resistance (corrosion-resistant members) have been formed of a ceramic, such as sintered alumina.
Recently, sintered yttrium aluminum garnet (YAG) has received attention as a sintered ceramic having a corrosion resistance higher than that of sintered alumina. Although the sintered YAG has a corrosion resistance higher than that of sintered alumina, however, the sintered YAG generally has much lower mechanical properties, such as the bending strength and the fracture toughness, than sintered alumina. The sintered YAG is therefore difficult to apply to corrosion-resistant members that require excellent mechanical properties as well as a high corrosion resistance. Thus, sintered bodies that contain alumina and YAG and have mechanical properties superior to that of sintered YAG and a corrosion resistance higher than that of sintered alumina have received attention as corrosion-resistant members (see, for example, Patent Documents 1 to 4).
For example, Patent Document 1 discloses a ceramic composite that has a uniform sea-island structure containing a polycrystalline α-Al2O3 as the sea and polycrystalline YAG as the island. This ceramic composite has no colony and has a three-point bending strength of at least 500 MPa at 1500° C. in the air. Such a ceramic composite may be formed as follows: first, an α-Al2O3 powder and an Y2O3 powder are dry-blended or wet-blended at a desired ratio to prepare a mixed powder, the mixed powder is then melted in a known melting furnace, such as an arc melting furnace, at a temperature, for example, in the range of 1800° C. to 2500° C., and the melt is directly poured into a crucible, and is solidified in one direction to form the ceramic composite described above.
Patent Document 2 discloses a plasma-resistant sintered alumina. A main component Al2O3 has a particle size in the range of 10 to 40 μm, and YAG in the Al2O3 has an average grain size in the range of 0.1 to 1 μm. The number of YAG crystal grains is at least 20 in a 10 μm×10 μm area. This plasma-resistant sintered alumina is formed by molding and firing a raw material containing 100 parts by weight of Al2O3, 1 to 10 parts by weight of Y compound in terms of Y2O3, and 0.01 to 0.1 parts by weight of Mg compound in terms of MgO. The raw material is heated to 1600° C. at a heating rate in the range of 10° C. to 100° C./h, and is fired at a temperature in the range of 1600° C. to 1850° C. in a reducing atmosphere. The Y compound is an yttrium oxide precursor, such as yttrium chloride, yttrium acetate, or yttrium nitrate. The Mg compound is magnesium sulfate or magnesium nitrate. Use of these compounds allows Y or Mg to disperse well in the alumina composition, can prevent the selective corrosion of Al2O3 and deterioration in mechanical properties of a sintered body, and can improve the plasma resistance.
Patent Document 3 discloses a high-strength sintered alumina that contains 50% to 97% by weight of alumina and 3% to 50% by weight of YAG. The alumina has an average grain size in the range of 2 to 10 μm. The YAG has an average grain size in the range of 1.5 to 5 μm. The ratio of the average grain size of the alumina to the average grain size of the YAG is more than one and less than seven. The high-strength sintered alumina is produced by firstly mixing 50% to 97% by weight of alumina powder and 3% to 50% by weight of YAG powder. The alumina powder has a purity of at least 95%, an average particle size in the range of 1 to 15 μm, and a BET specific surface area in the range of 1 to 4 m2/g. The YAG powder has an average particle size in the range of 0.6 to 1.2 μm and a BET specific surface area in the range of 2 to 5 m2/g. The mixed powder is then mixed with an organic binder. The mixed powder is then granulated, molded, and fired to form the high-strength sintered alumina.
Patent Document 4 discloses a high-strength and high-hardness alumina ceramic that contains 0.5% to 12% by weight of YAG particles and the remainder substantially composed of alumina. YAG crystals have an average grain size in the range of 0.05 to 1.5 μm. The alumina has an average grain size in the range of 0.5 to 5.0 μm. YAG crystal grains are dispersed within a grain boundary of a sintered body and within an alumina particle. The alumina ceramic may be formed as follows: first, a water-soluble aluminum salt and a water-soluble yttrium salt are dissolved in water to prepare a solution, an alumina powder is then added to the solution, the solution is subjected to a neutralization reaction with ammonia to form a mixed powder composed of Al—Y hydroxide and alumina, the mixed powder is then calcined at a temperature in the range of 300° C. to 1000° C. to produce an alumina powder in which YAG particles are dispersed, and the alumina powder is granulated, molded, and fired to form the alumina ceramic described above.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-81257    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-37660    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-255634    Patent Document 4: Japanese Unexamined Patent Application Publication No. 11-335159